Polymers with super-absorbing capacity are frequently used within various technical fields and for various applications. Some of the most common super-absorbents are polyacrylic acid sodium salts and carboxymethyl cellulose (CMC) sodium salts. These are frequently used in absorbing materials in the form of powders, and often in combination with cellulose fibres, etc. For many applications it is beneficial to have the super-absorbent polymer in another form than powder, since powder has a tendency to migrate and escape, thereby generating dust. Another problem with super-absorbents in powder form is gel blocking. Gel blocking means that the powder agglomerates and when the agglomerate comes in contact with polar liquid a gel layer is formed on the surface of the agglomerate. The gel layer on the surface of the agglomerate prevents the liquid from reaching the super-absorbents in the center of the agglomerate. One way to overcome the problems associated with super-absorbing powder is to form fibres from the polymer. Fibres of polyacrylic acid sodium salt can be made by dry spinning of low molecular weight polymer and subsequent cross-linking of the dried fibre. The absorbency and thickening, gelling effect in polar liquid tend to be lower for those fibres compared to the powder form, as a result of insufficient cross-linking. In various applications, like wound fillers, there is a reluctance to use polyacrylic acid sodium salts due to the risk of allergic reactions caused by residual monomers and by-products from for instance the initiators.
Absorbing materials based on sodium calcium alginate and/or sodium CMC constitute archetypical polymeric materials within wound care. Fibres of sodium calcium alginate can be formed by wet spinning of a solution (dope) of sodium alginate that is pumped into a coagulation bath via small orifices (a spinneret). In the coagulation bath an ion exchange between calcium ions and sodium ions take place and “alginate fibres” are formed. A drawback with this process is that it is difficult to make soluble fibres since the calcium ion forms a cross-link between the molecules that hardly can be reversed unless complexing agents are used. The process used to form alginate fibres is described in EP 476 756. Other drawbacks of the wet spinning process are that it is slow and requires several steps of drying, etc.
Fibres that dissolve and gel are useful in order to maximize liquid absorption. Fibres which do not dissolve absorb a certain amount of liquid but residual liquid may remain outside the fibres. When fibres dissolve they will form a hydrogel, which exhibit advantages relating to absorption of larger amounts of liquid in the gel. To overcome the disadvantage of non-dissolving fibres, such as alginate fibre, fibres based on CMC was developed. CMC is not thermoplastic so the production has to be performed by modification of cellulose fibres. In the case of Aquacel (a commercial product from ConvaTec) a regenerated cellulose fibre such as Lyocell is used as starting material. The cellulose fibre is processed chemically to form a sodium CMC fibre, as disclosed in U.S. Pat. No. 4,410,694, WO 93/12275 and WO 94/16746. A disadvantage pertaining to this procedure is the inflexibility of the process and the sensitivity of the ionic groups in the polymer against dibasic cations such as calcium ions, which are often present in body fluids.
In practice it is often beneficial to balance the solubility of the fibres so that it is just on the border between full dissolution and not being dissolved. This can be achieved either by physical or chemical cross-linking between the polymer strands. If the number of cross-links is too high it will lead to a fibre with poor absorption capacity. If on the other hand the number of cross-links is too low this can lead to a weak gel, depending on the type, concentration and molecular weight of the polymer. By chemical cross-linking is meant a covalent bond between two or more polymer strands. Physical cross-linking can be achieved by crystalline parts, hydrogen bonding, ionic groups that interact or by hydrophobic interactions. A combination of the previously mentioned interactions is also possible.
In U.S. Pat. No. 6,750,163 a way of making melt processable polyethylene oxide fibres is described. The purpose of these fibres is to be easily dissolvable but they cannot be used to form strong gels at low concentrations.
Further, U.S. Pat. No. 6,017,625 discloses a method of making water-absorptive polyurethane fibres. Explicit examples show fibres prepared by using MDI methylene bis(p-phenyl isocyanate) (MDI) together with polyethylene glycol (PEG).